and Warren Buffett's diversified Berkshire Hathaway -- collectively hauled in more than half the total net income reported by U.S-based companies in the stock index last year, according to a USA TODAY analysis of data from S&P Global Market Intelligence. The analysis includes 462 U.S.-based companies that have reported their net income for calendar year 2015.

Seeing such a small group of big companies generate such a high percentage of the nation's total corporate net income is noteworthy, as it shows how profits have gotten even more concentrated. In 2014, 52 S&P 500 companies generated half the overall corporate profit within that index.

"You have a concentration of business in the hands of a small number of players," says Jack Ablin, chief investment officer of BMO Private Bank.

This shift of profit into the hands of a shrinking group of companies is a danger for investors, who are now counting on fewer companies to deliver to keep the markets rising, Ablin says.

"It's the opposite of diversification," he says. "You're now concentrated."

These companies also have greater financial resources to sway lawmakers to make policy decisions that benefit them more, Ablin says."These largest companies have (outsized) influence on lawmakers to enact certain financial to protect their business perhaps at cost of smaller competitors."

Total net income in 2015 fell 15% among the 462 U.S. companies that have reported their calendar-year results, based on data from S&P Global.

Much of the drop is due to massive declines in profit from oil companies, including a $23 billion net loss from energy exploration company Apache

. This analysis uses companies' reported net income, which includes all charges and gains and does not strip out unusual items.

Yet even when results are adjusted and unusual gains and losses are left out, 2015 was a down year. Total adjusted profit in the S&P 500 has declined 0.7% excluding unusual charges that Wall Street analysts typically ignore, S&P Global says.

Apple continues to be the biggest contributor, bringing in 6.7% of the total net income reported by U.S.-based companies. That's up from the 4.7% of total U.S. net income the company hauled in during 2014. That's because Apple's profit rose 21% in 2015 and it made more money than any other U.S.-based company in the S&P 500 at $53.7 billion in net income. Enormous profit margins on the company's smartphones continue to be a business model that no other U.S. company has been able to match.

Big banks are also massive generators of net income, and that trend that continued in 2015. JPMorgan hauled in $24.4 billion in net income last year -- second only to Apple -- giving it a 3% piece of the nation's total net income. Five of the biggest generators of profit in the S&P 500 are financial firms, including Berkshire Hathaway.

Hopes are high that total profits from companies in the Standard & Poor's 500 will grow in 2016. Analysts expect adjusted earnings of all S&P 500 companies to rise 2.2% in 2016, says S&P Global.